1. Field of the invention:
This invention relates to a solid-state image sensor, especially an interline CCD image sensor (i.e., an interline coupled charge device image sensor), which can be made compact and which can suppress both blooming and smear.
2. Description of the prior art:
Although solid-state image sensors are advantageous over pick-up tubes in compactness, minimization of the weight, durability, reliability, exhibiting no burn-in from intense overloads, etc., they give rise to blooming and smear which do not arise in pick-up tubes. Blooming occurs when excess signal charges in the photodiode overflow into the transfer channel, resulting in distinct white lines on the obtained image, while smear takes place when a signal charge generated deep inside the substrate diffuses into the transfer channel, resulting in indistinct white lines on the obtained image. Blooming can be eliminated by means of overflow drains composed of an n.sup.+ -diffusion layer which absorb excess charges, as disclosed by W. F. Kosonocky et al., "Control of Blooming in Charge-Coupled Imagers", RCA Review, vol. 35, pp 3-24, March 1974. However, the problem of smear has not yet been solved.
The above-mentioned approach for the elimination of blooming sacrifices photosensitivity and dynamic range because the active area is reduced, and accordingly a method for the transfer of the excess charges into the substrate has been proposed by Y. Ishihara et al., "Interline CCD Image Sensor with an Antiblooming Structure", IEEE Transactions on Electron Devices, vol. ED-31, No. 1, January 1984. FIG. 3 shows a unit cell of the interline CCD image sensor described in the above-mentioned article, wherein an n.sup.- -layer 3 constituting a buried channel of the vertical CCD register is formed on a thick p-type layer 13, while an n-type layer 14 constituting a p-n junction photodiode as a photosensitive area is formed on a thin p-type layer 12. Both the thick p-type layer 13 and the thin p-type layer 12 are positioned on an n-type substrate 1. A transfer gate region 7 positioned between the n.sup.- -layer 3 and n-type layer 14 contains the part of the p-type layer which has not undergone depletion. This unit cell is electrically isolated from adjacent unit cells by channel stops 6 composed of p.sup.+ -layers formed in the surroundings thereof. On the n.sup.- -layer 3 and the transfer gate region 7, a polysilicon electrode 9, which is an electrode for the vertical CCD register to be driven by pulses .phi.v fed to the electrode 9, is formed. On the polysilicon electrode 9, an Al film 10 is further formed as a photo-shield.
A reverse bias voltage is applied between the channel stops 6 and the n-type substrate 1, resulting in a complete depletion of the thin p-type layer 12.
The pulses .phi.v have three levels, V.sub.H (high), V.sub.M (middle) and V.sub.L (low). When the pulses .phi.v are between the V.sub.M and V.sub.L levels, the n.sup.- -layer 3 constituting one half of one stage of the vertical CCD register area is isolated from the n type layer 14 constituting the photosensitive area by the transfer gate region 7 containing a part of the p-type layer. When the pulses .phi.v are at the highest potential V.sub.H, a signal charge is accumulated into the photosensitive area and transferred from the photosensitive area into the vertical CCD register area through the transfer gate region 7, and the potential of the n-type layer 14 constituting the photosensitive area results in a level equal to the surface potential of the transfer gate region 7. This signal charge is further transferred to a horizontal CCD register (not shown) to be read out at a gated charge detector (not shown).
As the photogenerated signal charge is accumulated into the n-type layer 14, the potential of the n-type layer 14 decreases. The excess charges generated in the n-type layer 14 receiving intense illumination thereto are transferred from the n-type layer 14 into the n-type substrate 1 through the thin p-type layer 12 which has been depleted, resulting in a suppression of blooming.
As mentioned above, the conventional image sensor shown in FIG. 3 can suppress blooming, but it still has the following serious problems:
The first problem is that the image sensor cannot be made compact because of the transverse expansion of the thick p-type layer 13. The size of the unit cell cannot be minimized beyond a certain extent in the transverse direction because the size of the thin p-layer 12 is essential to a certain extent.
The second problem is that smear takes place unavoidably when a signal charge generated in the non-depletive thick p-type layer 13 diffuses into the vertical CCD register and mixes with the signal charge in the vertical CCD register.